Alkylaromatic fluids have been proposed for use as certain types of functional fluids where good thermal and oxidative stability are required. For example, U.S. Pat. No. 4,714,794 (Yoshida) describes the monoalkylated naphthalenes as having excellent thermal and oxidative stability, low vapor pressure and flash point, good fluidity and high heat transfer capacity and other properties which render them suitable for use as thermal medium oils. The use of a mixture of monoalkylated and polyalkylated naphthalenes as a base for synthetic functional fluids is described in U.S. Pat. No. 4,604,491 (Dressler) and Pellegrini U.S. 4,211,665 and 4,238,343 describe the use of alkylaromatics as transformer oils.
The alkylated naphthalenes are usually produced by the alkylation of naphthalene or a substituted naphthalene in the presence of an acidic alkylation catalyst such as a Friedel-Krafts catalyst, for example, an acidic clay as described in Yoshida U.S. Pat. No. 4,714,794 or Dressler U.S. Pat. No. 4,604,491 or a Lewis acid such as aluminum trichloride as described in Pellegrini U.S. Pat. No. 4,211,665 and U.S. Pat. No. 4,238,343. The use of a catalyst described as a collapsed silica-alumina zeolite as the catalyst for the alkylation of aromatics such as naphthalene is disclosed in Boucher U.S. Pat. No. 4,570,027. The use of various zeolites including intermediate pore size zeolites such as ZSM-5 and large pore size zeolites such as zeolite L and ZSM-4 for the alkylation of various monocyclic aromatics such as benzene is disclosed in Young U.S. Pat. No. 4,301,316.
In the formulation of functional fluids based on the alkyl naphthalenes, it has been found that the preferred alkyl naphthalenes are the mono-substituted naphthalene since they provide the best combination of properties in the finished product: because the mono-alkylated naphthalenes posses fewer benzylic hydrogens than the corresponding di-substituted or polysubstituted versions, they have better oxidative stability and therefore form better functional fluids and additives. In addition, the mono-substituted naphthalenes have a kinematic viscosity in the desirable range of about 5-8 cSt (at 100.degree. C.) when working with alkyl substituents of about 14 to 18 carbon atoms chain length. Although the mono-alkylated naphthalenes may be obtained in admixture with more highly alkylated naphthalenes using conventional Friedel-Krafts catalysts such as those mentioned above or by the use of zeolites such as USY, the selectivity to the desired mono-alkylated naphthalenes is not obtained.
Zeolite catalysts have been found to be effective for the production of mono-alkylated naphthalenes, as disclosed in U.S. Pat. No. 4,301,316 and, more recently, U.S. Pat. No. 4,962,256. Good selectivity for the preferred mono-substituted naphthalenes may be obtained by the incorporation of cations having a radius of at least 2.5 A in large pore size zeolites such as zeolite Y, as described in Ser. No. 07/505,392 now U.S. Pat. No. 5,034,563.